The invention relates to an exhaust gas turbocharger for an internal combustion engine and a method for operating an exhaust gas turbocharger with a turbine which is disposed in an exhaust duct and a compressor which is disposed in an engine intake duct and operatively connected to the turbine by a shaft which is rotatably supported by a bearing housing and wherein an electric motor is integrated into the bearing housing of the exhaust gas turbocharger and includes a rotor which is disposed co-axially with the shaft.
Such an exhaust gas turbocharger is disclosed in U.S. Pat. No. 6,023,452. The exhaust gas turbocharger includes an electric motor, which is energized during low load and speed operation of the engine for driving the charger to increase the speed of the turbocharger. The electric motor is arranged on the shaft between the compressor and the turbine and is firmly connected to the shaft for rotation therewith. Upon energization of the electric motor, a driving force is applied to the shaft and the compressor by the electric motor in addition to that applied by the turbine so that already at low engine load and speed an increased charge air pressure can be generated for increasing the engine power output.
With such an exhaust gas turbocharger, the compressor out-put can be increased by the electric motor in a controlled manner. However, every time the electric motor is activated, it is also necessary to accelerate also the turbine shaft and the turbine wheel together with the impeller of the compressor. These components, that is, the turbine wheel and the turbine shaft, however have generally a substantially higher moment of inertia than the impeller of the compressor and therefore require a high electric energy input for their acceleration by the electric motor. Because of the high moment of inertia, the exhaust gas turbocharger also has a slow response behavior under transient operating conditions. To improve the response of such an exhaust gas turbocharger, a large electric motor is required which however further increases the moment of inertia and, furthermore, has high electric power requirements.
It is therefore the object of the present invention to provide an exhaust gas turbocharger with an auxiliary electric motor drive, which has relatively low electric power requirements and with which the transient behavior of the exhaust gas turbocharger is improved.
In an exhaust gas turbocharger for an internal combustion engine having an air intake duct and an exhaust duct and wherein the turbocharger includes a compressor arranged in the air intake duct and a turbine arranged in the exhaust duct and a shaft carrying the turbine wheel and the compressor impeller, an electric motor is provided which includes a rotor provided with an air mover disposed in the inlet area of the compressor and being controllable for rotation independently of the rotation of the turbine.
The air mover forms a component additional to the compressor impeller and can be operated independently of the turbine wheel. The rotor of the electric motor drive operates the air mover. However, since there is no rigid connection in the motion transmission path between the air mover and the turbine wheel, the electric motor and, together therewith, the air mover can be operated independently of the movement of the turbine wheel. The movement of the rotor and the movement of the turbine are uncoupled. This has the advantage that, upon operation of the electric motor, the turbine wheel with its relatively high mass does not also have to be accelerated so that the energy requirements for the electric motor are relatively small.
With the air mover, the compressor can pump more combustion air and the air can be compressed to a higher charge air pressure, with which it is supplied to cylinder inlets of the engine. The rotor of the electric motor may be a component of the air mover, it may even be identical with the air mover. However, the rotor of the electric motor may also be a separate component which is connected with the air mover for rotation therewith.
The air mover may form an integral part with the compressor impeller or with a part of the compressor impeller so that the air mover and the impeller are firmly interconnected for rotation in unison. In that case, the motion transfer path between the compressor impeller and the turbine wheel can be interrupted that is the movement of the compressor impeller including the air mover can be uncoupled from the movement of the turbine wheel.
It may, however, be advantageous to permit a relative rotational movement of the air mover with respect to the compressor impeller. In that embodiment, the rotor takes over the function of moving air through the compressor in addition, or in the alternative, to the compressor impeller. The movement of the air mover and of the compressor impeller can be uncoupled. There is basically an additional movement interruption possible in the motion transmission path between the compressor impeller and the turbine wheel.
The air mover is arranged preferably upstream of the compressor wheel in the inlet area of the compressor so as to form a pre-stage for the compressor wheel.
In an advantageous embodiment of the invention, the direction of rotation of the electric motor is reversible so that the electric motor can have a direction of rotation in the same sense as the turbine wheel or in the opposite sense. In this way, the work to be performed by the turbine can be supported by the electric motor when energized in the same direction of rotation as the turbine. When the electric motor is energized in the opposite direction, the air mover can be employed as a throttle, whereby a blocking effect occurs at the compressor inlet. Depending on the circumstances, even a vacuum can be generated at the air intake of the internal combustion engine by the air mover so that, in principle, a throttle valve in the air inlet duct of the internal combustion engine is not needed.
In the transmission path between the air mover and the urbine wheel, a clutch is preferably provided. In a first possible embodiment, the clutch is arranged between the air mover and the compressor impeller and, in a second possible embodiment, the clutch is disposed between the compressor impeller and the shaft or, respectively, between the turbine wheel and the shaft, wherein also a combination of the two possibilities is possible. A clutch arranged between the air mover which is in the form a pre-stage impeller (pre-charger) and the compressor impeller in the form of a separate component has the advantage that, with disengaged clutch, only the mass of the rotor and of the air mover have to be accelerated, whereas the masses of the compressor impeller, the shaft and the turbine wheel do not need to be accelerated by the electric motor. The masses to be driven by the electric motor can therefore be kept very small. With the air transport by the pre-charger, an increased intake air pressure can be achieved even when the compressor wheel is at a standstill or rotates at only very low speed. As soon as the compressor speed reaches the rotor speed of the electric motor, the clutch between the air mover and the compressor wheel can be engaged so as to rigidly interconnect the two components whereby the air mover is operated without energization of the electric motor. The energy needed for driving the charger/compressor is then provided exclusively by the exhaust gas back-pressure.
In the embodiment, in which a clutch is provided between the compressor impeller and the shaft or, respectively, between the shaft and the turbine wheel, the impeller can be disengaged from the turbine wheel. This embodiment is desirable for the arrangement, in which the air mover is integrated with the compressor impeller or is rigidly connected thereto for rotation therewith and wherein the air mover and the compressor impeller are to be movable independently of the turbine wheel.
In another advantageous embodiment of the invention, the clutch operating mechanism is coupled with the energization of the electric motor in such a way that the clutch is disengaged when the electric motor is energized, and is again automatically engaged when the electric motor is de-energized. With such a functional interconnection of the clutch operating mechanism and the energization of the electric motor, it is ensured that the least possible mass is to be accelerated by the electric motor upon its energization.
Various embodiments of the invention will be described below in greater detail with reference to the accompanying drawings.